Let him have his fit, don't pull back on his mouth, just kick him on and stay relaxed. Let him get it out and show him there is no purpose to behaving like that. Once he's behaving keep going with you're work like nothing happened. Eventually he will learn that there is no point in fighting, he's going to have to work with you either way
Ride with a crop. Not to hurt him. Just to let him know that you mean business. As soon as he starts to back up and stop tap him up with the crop until he goes forward. Do this with him Everytime you ride and eventually he will stop. He is doing this to test you because horses always need someone in charge and right now he thinks he is in charge and can do whatever he wants. You have to tell him that you are in control. Hope this helps.
Maybe make sure you don't have too much contact on his mouth. Maybe when you driest get on have a long reign, let him walk around, stretch and relax, then slowly pick up your reigns and ask for a trot at the same time, try to think forwards and into contact rather that pulling into a frame as this may confuse him into thinking you want downwards transitions. Maybe every time he goes backwards make him go forwards and allow with your reigns or if he keeps backing up turn him in a circle until he stops, the ask forwards, if he doesn't then repeat the circle. Maybe if none of this works when you start to warm up just go for a big canter and think forwards and then make him walk and then exercise as normal?